286 
DR. MANTELL ON THE OSTEOLOGY OF 
The posterior surface of the body of the axis must be deeply concave, as we shall 
presently see. 
The large cervical vertebrae from the Wealden strata, with reversed convexo-concave 
joints {Streptospondylus major, O.), (Plate XXVIII. fig. 4), enter into the composition 
of the cervical region of the spinal column of the Iguanodon. We are forced to this 
conclusion by the following circumstances: — 1st, an anterior dorsal vertebra (Plate 
XXVIII. fig. 5.) from the same deposits, with similar but less marked deviations in the 
form of the articular facets, and with a configuration of the neural arch, so far as per- 
fect, identical with that existing in more posterior dorsal vertebrse with plano-concave 
joints, well-recognized as belonging to this great herbivorous reptile, links together 
these apparently discrepant vertebral types : 2ndly, the amount of variation here 
assumed is parallel to that which exists in its affine among the Crocodilidae, the 
Steneosaurus rostro-minor* ; and similar changes in the form of corresponding arti- 
cular facets occur in the spinal column of the Ruminants, Solipeds, and other Pa- 
chyderms : Srdly, other alterations in the sculpturing of the neural arch of equal 
value with the modifications in the form of the articular aspects of the body, are con- 
comitant with these changes in the different vertebrae just mentioned, and are 
equalled in kind and degree by those which occur in the series of neural arches of 
the spine in the recent Crocodiles: 4thly, these convexo-concave cervical vertebrae 
are found in such collocation with other well-determined bones and vertebrae of the 
Iguanodon as to leave no reasonable doubt of their belonging to that animal : 
5thly, the number of these vertebrae of different ages and sizes in our collections is 
such as we might have expected on that supposition ; and 6thly, if these be not the 
cervical vertebrae of the Iguanodon, we have the (assumed) Streptospondylus major 
with nothing but a neck, whilst the Iguanodon, as yet known, is wholly destitute of 
that region of the spine : is it not, therefore, more probable that the neck of the so- 
called Streptospondylus belongs of right to the Iguanodon, especially as the bones of 
that reptile, tested by the fortunate discovery of the Maidstone specimen, constitute 
the great majority of the osseous relics from the deposits of the Weald ? in other 
words, the Iguanodon is the reptile par excellence characteristic of the Wealden 
formation. 
The Streptospondylian form of the body of a vertebra can no more characterize 
a genus of Reptiles than the Amphicoelian or Coelospondylian modifications ; each is 
common to a group of species constituting not only distinct genera and families, but 
also orders and subclasses. Nay, the Streptospondylian type is not even persistent 
throughout the elements of the same spinal column ; it disappears towards the 
middle of the dorsal region in the Steneosaurus rostro-minor, the best known example 
of this structure, and that in which it was first recognized by Baron Cuvier. The 
genus Streptospondylus of V. Meyer ought therefore to be abolished, and the resi- 
dual generic appellation Steneosaurus (G. St. Hilaire) be retained to designate Cu- 
* Vide Cuvier, Oss. Fossiles, vol. ix. 8vo edit. 
